It is known that an otherwise untreated web of paper or paperboard rawstock will react adversely to the application of a liquid by becoming rougher, since the surface fibers of the web absorb moisture leading to fiber swelling, breaking of hydrogen bonds and fiber reorientation. If the liquid application is in the form of a coating or the like, the web may actually become smoother, because of the presence of smoothing ingredients in the coating, but the improvement in smoothness is not as great as would be expected because the surface of the rawstock beneath the coating becomes roughened in the presence of the liquid phase of the coating. Because of this phenomenon, and because one of the objectives in the production of paper and paperboard is to produce a smooth finish, it is desirable to smooth the surface of the rawstock web before coating.
One of the known techniques for at least partially smoothing the surface of a web of paper or paperboard rawstock is to finish the web. This may be accomplished by calendering the web using a machine calender on a papermachine. The degree of finish obtained by this method depends upon the nip pressure reached in the calender. However, the smoothness obtained by machine calendering is short lived when the web is subsequently brought into contact with water or a composition containing water, since dry cellulose fibers will not bond under pressure. On wetting, there will be substantial recovery due to fiber swelling and release of stress. An improvement in this technique is to calender the web while it is wet. The compressing of plasticized cellulose fibers will consolidate the web without undue stress, and the surface achieved will retain a substantial amount of its integrity for a longer time after wetting than is common for a web that is compressed in an essentially dry state. Conventional processes for wet calendering employ either a water box on the calender, water sprays, or steam showers. Unfortunately the use of these methods is haphazard at best and difficult to control, so that the results are not predictable. In most cases, the quantity of moisture transferred to the web is uneven, too great or too small, resulting in unsatisfactory results. Nevertheless, recent innovations in the paper industry have provided equipment that is capable of applying controlled amounts of water to a web in a uniform manner, and it was the introduction of this equipment that made the present invention possible.